The present invention relates to a board machine and method for manufacturing a multilayer cardboard web with a printable surface layer, comprising a wet section and a press section, which wet section includes a first forming unit for forming a first layer, which first forming unit has at least one forming wire, running in an extended loop up to the press section to form a pick-up point for the multilayer cardboard web, and one or more further forming units for forming one or more further layers and for couching the same with said first layer on said extended forming wire of the first forming unit to form the multilayer cardboard web, which press section includes at least one double-felted press, having an upper press element, a lower press element in the shape of a press roll, which press elements create a press nip with each other, an upper press felt, running in a loop around a plurality of guide rolls and a pick-up roll, arranged at said pick-up point for transferring the multilayer cardboard web to the upper press felt, and a lower press felt, running in a loop around a plurality of guide rolls.
As used herein, the expression xe2x80x9cthe 0 line of the pressxe2x80x9d is defined, for a roll press, as the tangent perpendicular to a straight line intersecting the centers of the press rolls and, for a shoe press, as the tangent of the transition from the concave curvature to the convex curvature of the shoe at the exit of the press nip.
One side of a multilayer cardboard web is often used for printing. This side, denoted the front side of the finished cardboard product, is formed by a surface layer that must have a high degree of surface smoothness to provide good printability. Special pulps are used for manufacturing the surface layer. Short-fiber pulps result in surface layers with improved printability. The pulp intended for the printable surface layer is preferably, but not necessarily, bleached. It may consist of a mixture of short-fiber and long-fiber pulps, in which the short-fiber proportion of the pulp may constitute 50-70 per cent by weight of the mass. However, the short-fiber proportion may constitute 100 per cent. The layer to be printed may also be made of 100 per cent bleached long-fiber pulp. Short-fiber pulp can be pulp from birch or eucalyptus, for instance, while long-fiber pulp can be pulp from pine, for instance.
A number of methods and machines for manufacturing multilayer cardboard webs are described in patent literature and the following are mentioned by way of example: EP-0 511 186, WO 92/06242, U.S. Pat. No. 4,961,824, EP-0 511 185, U.S. Pat. No. 5,074,964, EP-0 233 058 and SE-506 611.
U.S. Pat. No. 5,639,349 (corresponding to DE-4401761) describes a method for improving the quality of multilayer papers in the wet section of a paper machine by recirculating the drainage water within each forming unit. The outer layer of the paper web is made of stock of higher quality than the stock for the core. The patent specification does not mention cardboard or board and the problem associated with providing a printable surface layer on a multilayer cardboard web. Neither does the patent specification touch upon the problem relating to the press section and the web run in the same, and in particular does not address the problem of pressing of a multilayer cardboard web with a printable surface layer.
U.S. Pat. No. 4,957,778 describes a paper machine for manufacturing two-layer carbonless copy paper. The paper machine has upper and lower fourdrinier formers, the layers of which are combined by couching to form a coherent paper web, which is pressed in a press with two single-felted press nips, created by two press rolls and a counter roll shared by the same. Multilayer cardboard webs are not touched upon in this patent specification and, consequently, neither are the problems associated with pressing a multilayer cardboard web.
In practice, the predominant technique for manufacturing a multilayer cardboard web is to manufacture the surface layer with a forming unit, for instance an upper fourdrinier former, arranged relative to at least one other forming unit, for instance a lower fourdrinier former, in such a way that the surface layer is couched with a subjacent layer and the cardboard web emerges from the wet section with the surface layer facing upwards. This in turn dictates the configuration of the press section. In accordance with conventional techniques, a double-felted roll press is employed as the first press. It is also known to use a double-felted shoe press with the shoe in a top or bottom position as the first press. A first double-felted press of known kind has an upper felt acting as a pick-up felt to transfer the cardboard web to the press nip, while the lower felt is intended to carry the cardboard web subsequent to its passage through the press nip. The surface layer of the cardboard web thus comes into direct contact with the upper felt. Accordingly, to be able to satisfy the requirement of high surface smoothness of the surface layer, the structure of the web-contacting surface of the upper felt must not be too rough. If, on the other hand, the structure of the web-contacting surface of the lower felt were to be too smooth or fine to ensure the correct web run after the press nip, the lower felt will not be sufficiently open to allow permeation of water and will relatively quickly become clogged with fibers, which means that reconditioning of the lower felt cannot be accomplished with the desired result and that the service life of the lower felt becomes relatively short. In practice, the two contradictory requirements for the properties of the upper felt and the lower felt result in the requirement that the differences between their surfaces structures with respect to roughness or smoothness become relatively small and there is, therefore, a risk of the cardboard web sometimes having a tendency to accompany the upper felt after the press nip instead of the lower felt as intended, even if the lower felt has the smoother surface. To ensure the correct web run in a shoe press with the shoe in the bottom position the lower felt must be passed over the downstream edge of the shoe and the upper felt passed approximately in the direction of the so-called 0 line, but this is not an acceptable solution as the web is then subjected to detrimental shear forces during its passage over the shoe edge.
The object of the present invention is to provide an improved board machine and an improved method of manufacturing a multilayer cardboard web. The invention thus enables the manufacture of a multilayer cardboard web having a printable surface layer with a desired high degree of surface smoothness and maximum dry-solids content after the press section, while safeguarding the web run in the press section.
The board machine, in accordance with the invention, is characterized in that the first forming unit is arranged to form the printable surface layer and arranged with its extended forming wire to transfer the multilayer cardboard web to the upper press felt of the press with the printable surface layer facing downwards to contact the lower press felt in the press nip. The lower press felt has a finer web-contacting surface to exert a greater adhesion force on the multilayer cardboard web than the upper press felt, and the lower press felt at the discharge side of the nip is arranged to encompass the lower press roll by a pre-determined minimum sector angle xcex1 measured from a point in the press nip intersected by the 0 line of the press, as defined herein for a roll press and a shoe press, respectively.
The method, in accordance with the invention, is characterized in that the printable surface layer is formed in the first forming unit and the extended forming wire transfers the multilayer cardboard web to the upper press felt of the press with the printable surface layer facing downwards so that it is in contact with the lower press felt in the press nip. The lower press felt exerts a greater adhesion force on the multilayer cardboard web than the upper press felt by virtue of its finer web-contacting surface, and the lower press felt is caused to encompass the lower press roll by a pre-determined minimum sector angle xcex1 measured from a point in the press nip intersected by the 0 line of the press, as defined herein for a roll press and a shoe press, respectively.